optical activity

Meaning: Optical activity is the ability of a chiral substance to rotate the plane of plane-polarized light. Clockwise rotation is dextrorotatory, (+) or d; counterclockwise is levorotatory, (−) or l. Enantiomers rotate light by equal amounts in opposite directions. A racemic mixture, with equal amounts of both enantiomers, shows no net rotation because the effects cancel. A meso compound has stereocenters but an internal mirror plane, so it is achiral and optically inactive.

Equation: [α] = α / (l × c), where [α] is specific rotation (° mL g⁻¹ dm⁻¹), α is observed rotation (degrees; SI unit rad), l is path length (dm; SI unit m), and c is concentration (g/mL; SI unit kg/m³).

On the MCAT: A polarimeter passage gives an observed rotation and asks for enantiomeric excess: ee = (observed [α] ÷ [α] of pure enantiomer) × 100%. Other questions ask which structures are inactive (racemic, meso, achiral) or what an SN1 reaction at a stereocenter does to rotation (it tends toward zero).

Common confusion: Students assume R means (+) and S means (−). The sign is measured experimentally and is not set by the R or S label, so an R compound can be either (+) or (−).